The present invention relates to a solid state imaging device and a production method for the same.
In the recent trend toward a compact solid state imaging device, there arises a problem of decreased photosensitivity of the solid state imaging device because the area of a light receiving portion is reduced as the device becomes compact. As a countermeasure against this problem, a solid state imaging device comprising a micro lens for collecting light on the light receiving portion has already been realized. The technique for forming a micro lens is indispensable in the production of a solid state imaging device at present.
Now, a conventional solid state imaging device will be described.
FIG. 8 is a sectional view for showing an exemplified configuration of the conventional solid state imaging device. In FIG. 8, a reference numeral 11 denotes a semiconductor substrate, a reference numeral 12 denotes photodiodes formed on the surface of the semiconductor substrate 11 so as to convert incident light into a charge, a reference numeral 13 denotes a first flattening layer for flattening the surface of the semiconductor substrate 11, a reference numeral 14 denotes a color filter formed on the flattening layer 13, a reference numeral 15 denotes a second flattening layer for flattening the level difference on the color filter 14, and a reference numeral 50 denotes micro lenses formed on the second flattening layer 15 so as to collect light on the corresponding photodiodes 12.
The first flattening layer 13 is formed by coating the semiconductor substrate 11 with a transparent film material in a desired thickness. The color filter 14 is formed by, for example, photolithography so as to correspond to the respective photodiodes 12. The second flattening layer 15 is formed by coating the color filter 14 with a transparent film material in a desired thickness.
Each of the micro lenses 50 is made of a phenol resin or the like, and is formed above the corresponding photodiode 12 in the shape of a hemisphere. The micro lens 50 has an appropriate height so that light entering the surface thereof can be efficiently collected on the corresponding photodiode 12.
The hemispherical shape of the micro lens 50 is formed through the following procedures: First, a lens resin is coated on the second flattening layer 15. Then, the lens resin is subjected to an exposing treatment using a lens mask and further to a developing treatment, thereby patterning the lens resin so as to be located in the position above each of the photodiodes 12. Then, the patterned photo resist is heated so as to be melted, thereby forming the hemispherical shape of the micro lens 50 by making use of the surface tension.
FIG. 9 is a plan view of the conventional solid state imaging device taking from the upward direction, wherein a reference numeral 50 denotes micro lenses, and reference letters x and y indicate a space between the adjacent micro lenses at the center and a space therebetween at the end, respectively.
FIG. 10 is a sectional view for showing another exemplified configuration of the conventional solid state imaging device. In FIG. 10, a reference numeral 11 denotes a semiconductor substrate, a reference numeral 12 denotes photodiodes, a reference numeral 13 denotes a first flattening layer, a reference numeral 14 denotes a color filter, and a reference numeral 15 denotes a second flattening layer, all of which are similar to those shown in FIG. 8. This solid state imaging device is different from that shown in FIG. 8 in an interlayer insulating film 16 formed between the semiconductor substrate 11 and the first flattening layer 13. The interlayer insulating film 16 has a smooth surface but has irregularity in accordance with the irregularity on the semiconductor substrate 11. The first flattening layer 13 is formed in order to flatten the irregularity on the surface of the interlayer insulating film 16.
However, the aforementioned solid state imaging devices and the production methods for the devices have the following problems:
In the solid state imaging device of FIG. 8, for the purpose of further improvement of the photosensitivity, it is desired to make a space S between the adjacent micro lenses 50 as small as possible so as to enlarge the light receiving area of each micro lens 50.
In the conventional production method, however, when the micro lenses 50 are formed by heating the patterned lens resin (hereinafter referred to as the lens patterns), the heating temperature is set at a temperature where the lens patterns are completely melted. Therefore, the melted lens patterns are forced out of the bottoms of the intended patterns.
Accordingly, as is shown in FIG. 9, the space x at the center between the adjacent micro lenses 50 is smaller than the space y at the end. As a result, even when the space x at the center is extremely minimized, the space y at the end cannot be extremely minimized. In this manner, there is a limit in enlarging the light receiving area of each micro lens 50.
Furthermore, in the case where the space between the adjacent lens patterns is too small, the adjacent lens patterns which have been forced out of the intended patterns due to the thermal dissolution come in contact with each other, and eventually the lens patterns flow out. This spoils the shape of each micro lens 50, resulting in a smaller surface area of the hemisphere portion as well as a smaller height. As a result, the light quantity collected on each photodiode 12 is decreased, so that the photosensitivity of the device is degraded.
Additionally, in accordance with the study by the present inventors, it has been found that higher photosensitivity can be attained by allowing each micro lens to have a larger height and minimizing a distance between the micro lens and a light receiving portion in the case where the light receiving portion has a small area.
However, since the lens patterns are completely melted in the conventional methods, it is impossible, due to the surface tension, to make the height H of the micro lens 50 larger than a half R of the bottom width of the micro lens 50 in the alignment direction of the light receiving portions. Accordingly, when a distance between the micro lens 50 and the photodiode 12 is small, it is feared that the micro lens 50 cannot be formed into an optimal shape for collecting the light.
Furthermore, when the interlayer insulating film 16 is formed on the semiconductor substrate 11 as in the solid state imaging device of FIG. 10, a distance between the micro lens 50 and the photodiode 12 is enlarged by the thickness of the interlayer insulating film 16. Therefore, even when the micro lens 50 has an optimal shape, there is the possibility of incident light not being collected on the photodiode 12 owing to the effects of scattering and the like. In particular when the refractive index of the interlayer insulating film 16 is larger than those of the first flattening layer 13 and the color filter 14, such a phenomenon becomes conspicuous, resulting in degrading the photosensitivity of the device.